1. Field of the Invention
The invention relates generally to the semiconductor power devices. More particularly, this invention relates to new configurations and methods for manufacturing improved drift region that achieves a sawtooth shaped electric field profile that for implementation in power semiconductor devices such as insulated gate bipolar transistors (IGBT), power MOSFETs, emitter switched thyristors and power diodes.
2. Description of the Prior Art
Conventional technologies to configure and manufacture vertical power devices have difficulties and limitations to further improve the performances due to the tradeoff between the forward blocking voltage and the on-state voltage drop. The performance is further limited by another trade-off between the switching speed and the on-state forward voltage drop for devices that undergo “conductivity modulation” during forward conduction. A reduced doping concentration in the drift region generally formed in an epitaxial layer of the semiconductor power device is required for the device to block high voltages. However, the presence of the low-doped region leads to higher resistance and reduced forward conduction and reduced power efficiency and increased heat generation. Bipolar devices such as IGBTs and emitter switched thyristors improve the on-state voltage drop by high level of minority carrier injection in the low-doped drift region during conduction. However, the drawback in this case is the degradation in the switching performance.
One of the major scopes for improvement in vertical power devices comes from improving the electric field profile in the drift region. A typical vertical power device has a triangular or trapezoidal electric field during a forward blocking mode depending on whether it has a non-punch through or a punch through design, respectively. However, these electric field profiles require a thicker than necessary drift regions. An ideal electric field profile that requires the minimum thickness of the resistive drift region is a rectangular shape. This can be observed in FIG. 5, which is explained in greater detail later in the disclosure.
FIG. 1 (Prior Art) shows the cross section of a conventional insulated gate bipolar transistor (IGBT). The IGBT is a semiconductor power device that combines the metal oxide-semiconductor (MOS) gate control and bipolar current flow mechanism. The functional features of both a metal-oxide-semiconductor field effect transistor (MOSFET) and a bipolar junction transistor (BJT) are combined in an IGBT. Performance features of IGBT are designed to achieve a higher current density than the MOSFETs and faster switching characteristics than the BJTs. For these reasons, IGBT devices are implemented for high power (>10 kW), low to medium frequency (up to 30 kHz) applications. However, the conventional IGBT as shown in FIG. 1 is still confronted with the technical limitations that the drift region electric field does not have the ideal rectangular shape. So, it requires a drift region thickness that is more than the minimum necessary for sustaining the blocking voltage.
Accordingly, there is a need to provide new configuration for a semiconductor power device with improved electric field in the drift region. It is desirable that the semiconductor power device implemented with the new and improved drift region structures for providing improved drift region electric field can achieve the same voltage blocking capability as a conventional semiconductor power devices, such as a conventional IGBT, with a significant reduction, e.g., 20% reduction, in the thickness of the drift region. It is further desirable to provide the improved semiconductor power device with thinner drift region to achieve improved forward voltage drop, and switching performance to broaden the applications of the semiconductor power devices including but not limited to IGBT, power diodes, power thyristors and power MOSFETs.